4.5.5 The D.C. Motor
Comprehensive Assessment: Paper 2 MCQ • Paper 4 Theory • Paper 6 Practical
A current-carrying wire is placed in a magnetic field as shown. Which rule is used to determine the direction of force on the wire?
What is the primary function of the split-ring commutator in a d.c. motor?
A student wants to decrease the speed of rotation of a simple d.c. motor. Which change would achieve this?
A beam of electrons travels horizontally from left to right in a region where there is a uniform magnetic field directed into the page. What is the direction of the force on the electron beam?
A rectangular coil carrying current is placed between two magnetic poles as shown. In which direction will the coil initially rotate?
⚠️ Examiner's Critical Traps
- 1.Fleming's Rules: LEFT hand for Motors (Force), RIGHT hand for Generators (induced current). Don't mix them up!
- 2.Electrons vs Conventional Current: Electron flow is OPPOSITE to conventional current. For negative charges, reverse the current direction or use opposite thumb.
- 3.Split-Ring Function: State "reverses current EVERY HALF TURN" - not just "reverses current". The timing is crucial for marks.
- 4.Turning Effect Keywords: Use "force in OPPOSITE directions" or "forms a COUPLE" - not just "force pushes it around".
- 5.Speed Increases: More turns, more current, stronger field ALL increase speed. Wire thickness alone does NOT (unless it changes resistance affecting current).
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